1. Field of the Invention
Embodiments of the present invention relate to electrophotographic image forming apparatus using a plurality of developing units.
2. Description of the Related Art
In general, existing image forming apparatuses employ an electrostatic recording method and an electrophotographic recording method. One of such methods is an intermediate transfer method. In an intermediate transfer method, a toner image is formed on a photosensitive drum. The toner image is primarily transferred onto an intermediate transfer member so as to overlap a previously transferred toner image in sequence and, thereafter, the toner images are secondarily transferred onto a transfer material at the same time. An intermediate transfer method has an advantage over a multiple transfer method in which toner images of individual colors are sequentially transferred from the photosensitive drum onto a transfer material in that image transfer may be stably performed onto a variety of transfer materials.
In image forming apparatuses that employ an intermediate transfer method, four color toner images, namely, yellow, magenta, cyan, and black toner images, formed on a photosensitive drum are sequentially primarily transferred onto an intermediate transfer member in the form of a belt or a drum. The four toner images transferred one on top of the other are finally secondarily transferred onto a transfer material in one go. However, since it is difficult to obtain a transfer efficiency of 100% when the toner image is secondarily transferred onto a transfer material, a small amount of toner remains on the intermediate transfer member after the toner image has been secondarily transferred. The toner remaining after secondary transfer is scraped off using a cleaning blade. Alternatively, the toner remaining after secondary transfer is recovered using a cleaning blade provided on a photosensitive drum and a simultaneous transfer and cleaning method (refer to, for example, Japanese Patent Laid-Open No. 2009-116130).
In apparatuses that perform secondary transfer after four color toner images has been transferred onto an intermediate transfer member, the operating condition for secondary transfer needs to be changed in order to efficiently perform the transfer when a special sheet, such as heavy paper, is used as a transfer material. For example, the speed of the intermediate transfer member and the speed of secondary transfer are reduced as compared with those for plain paper. In such a case, in order to prevent throughput degradation, an operation at a normal speed is performed until primary transfer is completed, and before the leading edge of a toner image formed on the intermediate transfer member reaches a secondary transfer portion, the speed of the intermediate transfer member is reduced.
Recently, in order to reduce the size of the image forming apparatus, some the image forming apparatuses have had a distance between a primary transfer portion and a secondary transfer portion that is smaller than the length of an image in the conveying direction. In such a case, when primary transfer of a toner image of a fourth color onto the intermediate transfer member is completed, the leading edge of the toner image has already passed through the secondary transfer portion. Accordingly, in order to perform secondary transfer after the speed has been changed, the image forming apparatus causes the formed toner image to pass through the primary transfer portion without performing transfer in the primary transfer portion. At that time, when the toner image formed on the intermediate transfer member passes through the primary transfer portion, scumming and reverse transfer may occur due to the electric field in the primary transfer portion. In order to prevent such a problem, when the toner image formed on the intermediate transfer member passes through the primary transfer portion, the electric field is reduced so as to be lower than that needed for image formation (refer to, for example, U.S. Pat. No. 5,640,645).
However, when the intermediate transfer member having a toner image formed thereon passes through the primary transfer portion, the toner on the intermediate transfer member may be reverse transferred onto the photosensitive drum. If the reverse transferred toner is not recovered by the cleaning unit of the photosensitive drum, the toner reaches a charging unit of the photosensitive drum. In the charging unit, the toner receives electrical discharge. Thereafter, the toner is transferred onto the intermediate transfer member again. Thus, an image defect, such as blotches or pitched dots, occurs. In existing technologies, reverse transfer may be prevented. However, existing technologies do not disclose prevention of an image defect caused by reverse transferred toner.